BMR Calculator

Basal metabolic rate (BMR) is the amount of energy your body spends in a day just to stay alive while lying still, awake, comfortably warm and having not eaten for about 12 hours. It covers the work of the heart, lungs, brain, liver, kidneys and every other cell that keeps running whether you move or not. For most adults BMR makes up the largest share of daily energy use, usually around 60% to 70%. Physical activity and digesting food add the rest.

Because true BMR has to be measured under strict laboratory conditions, calculators predict it from body size, age and sex. This one offers three widely used equations. The Mifflin-St Jeor equation, published in 1990, is the default because comparison studies, including a review by the Academy of Nutrition and Dietetics, found it to be the most accurate predictor for healthy adults of normal weight and with obesity. The original Harris-Benedict equation dates from 1918 and was revised in 1984 by Roza and Shizgal to fit newer data; it tends to give slightly higher numbers than Mifflin-St Jeor. The Katch-McArdle equation works from lean body mass rather than total weight, so it ignores sex and age but needs an accurate body fat percentage. It can be the better choice for lean, muscular people, whose high muscle mass makes the other formulas underestimate.

The result is shown in Calories (kilocalories) or kilojoules. The table multiplies BMR by standard activity factors to estimate total daily energy needs, from 1.2 for a sedentary person up to 1.9 for someone doing very hard exercise every day or a heavy manual job. That figure, rather than BMR alone, is the one to compare with how much you eat. Eating less than your BMR for long periods is generally not recommended, because it makes it hard to meet nutrient needs and encourages loss of muscle.

As a worked example, a 25-year-old man who is 5 feet 10 inches (177.8 cm) tall and weighs 160 pounds (72.6 kg) has a Mifflin-St Jeor BMR of 10 × 72.6 + 6.25 × 177.8 − 5 × 25 + 5, or about 1,717 Calories a day. If he exercises four or five times a week, his daily needs come to about 2,515 Calories. Any of these equations can miss an individual's real BMR by 10% or more, so use the number as an estimate and adjust it based on how your weight responds.

BMR Variables

Many factors push BMR up or down, which is why two people of the same height and weight can burn quite different amounts at rest:

• Muscle mass: muscle tissue uses more energy at rest than fat tissue, so people with more lean mass have a higher BMR. Strength training is one of the few ways to raise it.
• Age: BMR tends to fall gradually through adulthood, mainly because muscle mass declines and activity drops. A large 2021 study in Science found that metabolism adjusted for body size stays fairly stable from about age 20 to 60 before declining.
• Sex: men usually have a higher BMR than women of the same size because they tend to carry more muscle and less fat.
• Body size: taller and heavier people have more tissue to maintain and so burn more energy at rest.
• Genetics: inherited differences account for part of the variation between people that body size cannot explain.
• Hormones and health: an overactive thyroid raises BMR and an underactive one lowers it. Fever, infection, burns and recovery from injury all raise energy needs.
• Climate: the body spends extra energy to stay warm in the cold and, to a smaller degree, to cool itself in heat.
• Diet: long periods of very low calorie intake lower BMR as the body adapts to save energy, which can slow weight loss.
• Pregnancy and breastfeeding: both raise energy requirements to support the growing baby and milk production.
• Caffeine and nicotine: both cause a small, temporary rise in energy use, but they are not safe or effective ways to manage weight.

BMR Tests

Online equations are estimates. For a measured value, clinics, hospitals and some gyms use indirect calorimetry, which calculates energy use from the oxygen you breathe in and the carbon dioxide you breathe out while resting under a hood or through a mouthpiece. Measured results can differ noticeably from predicted ones.

Resting Metabolic Rate

Resting metabolic rate (RMR), also called resting energy expenditure, is measured under looser conditions than BMR: you do not have to fast overnight or stay in the lab, so it includes a little energy from recent digestion and movement. RMR is usually within about 10% of BMR, and the two terms are often used interchangeably. The Mifflin-St Jeor equation was in fact developed to predict resting energy expenditure.

Modern Wisdom

Popular advice promises many tricks to speed up metabolism, from eating many small meals to drinking ice water or taking special supplements. The evidence for most of these is weak, and their effects, if any, are small. Meal frequency does not meaningfully change total energy use when the calories are the same.

What does help is more straightforward. Building and keeping muscle through resistance training two or more days a week supports a higher BMR and better health as you age. Eating enough protein helps preserve muscle, especially while losing weight, and protein takes more energy to digest than fat or carbohydrate. Staying active throughout the day, walking more and sitting less, adds far more energy use than any supplement. Avoid crash diets: severe restriction causes muscle loss and metabolic adaptation, which makes weight easier to regain. A modest deficit, enough sleep and steady habits give better long-term results than trying to force metabolism up.

Reference

The equations used by this calculator come from these published studies:

• Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 1990; 51(2): 241–247.
• Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. American Journal of Clinical Nutrition, 1984; 40(1): 168–182.
• Harris JA, Benedict FG. A biometric study of human basal metabolism. Proceedings of the National Academy of Sciences, 1918; 4(12): 370–373.
• McArdle WD, Katch FI, Katch VL. Exercise Physiology: Nutrition, Energy, and Human Performance. Lippincott Williams & Wilkins.

Mifflin-St Jeor: BMR = 10W + 6.25H − 5A + 5 (men); 10W + 6.25H − 5A − 161 (women)

W

body weight in kg

H

height in cm

A

age in years

F

body fat as a fraction (Katch-McArdle)

Revised Harris-Benedict: men 13.397W + 4.799H − 5.677A + 88.362; women 9.247W + 3.098H − 4.330A + 447.593. Katch-McArdle: 370 + 21.6 × (1 − F) × W.

Frequently asked questions

Sources

• NIH NIDDK – Body Weight Planner
• American Journal of Clinical Nutrition
• Dietary Guidelines for Americans